The present invention relates to containers for radioactive counting. Such containers are particularly useful in analyses of the kind in which in a radioactive element or compound is partitioned between two phases and the proportion of the radioactivity in one phase is determined. In our British Pat. No. 1,411,382 we described a technique for carrying out this sort of analysis in which the radioactive concentration of one phase was measured while it was in contact with the second phase. This technique involves shielding one of the phases while the radioactive concentration of the other is counted.
Many radio-assays involve the following steps:
(I) The sample containing the compound to be assayed is mixed in an assay tube with a standard amount of a radioactively labelled version of the compound and the two are caused to compete for reaction with a standard amount, insufficient for complete reaction, of a specific reagent for the compound to be; and
(II) Either the fraction of the compound which is bound to the specific reagent, or the fraction which is not bound, is removed from solution.
The removal of one of the fractions from solution is normally effected by precipitation and in these cases the assay tube is centrifuged so as to concentrate the precipitate into a small volume at the bottom of the tube.
In radio-immuno assays, one alternative to precipitation is to use assay tubes in which the antibody is coated on the surface of the tube, the bound portion of the antigen thereby being removed from solution as the reaction proceeds. Such tubes have been described, for example, in U.S. Pat. No. 3,646,346. Such tubes may be used in the method of our aforementioned British Patent, provided that only a part of the surface of the tube is coated with the antibody, the coated portion being shielded during counting.